War memorials had to be bought - so who was included might depend on whether they were liked by the person footing the bill. Often the ones you see in town centres etc were supported by public subscriptions, and in some cases there was a set charge for having your loved one included.
Factory owners often paid for a memorial to the men lost from their workforce.
One of our local libraries has a memorial in marble going up the main staircase, and families had to pay 1s and 3d for each name. Quite a slice out of a poor family's budget - so many of the fallen went unrecorded.